1. Field of the Invention
The present invention relates to a steam turbine, more particularly, having an improved structure of a casing especially suitable for a low pressure turbine.
2. Related Art
A steam turbine is mainly composed of a high pressure turbine into which main steam is introduced, an intermediate pressure turbine into which reheated steam is introduced, and a low pressure turbine coupled with a condenser.
A low pressure casing as a pressure vessel of the low pressure turbine is ordinarily divided into upper and lower sections by a horizontal surface through which a rotor axis passes so that a rotor can be easily inserted and an assembly/disassembly performance can be improved. Flanges provided for the respective horizontal portions of the upper and lower sections are fastened with each other by bolts. In a exhaust chamber of the low pressure casing, atmospheric pressure acts on the outside surface of the exhaust chamber, and on the other hand, the exhaust chamber receives the atmospheric pressure from the outside surface side thereof because the inside of the exhaust chamber is made to a vacuum state, and a wrapper plate and end plates of the low pressure casing are deformed so as to be greatly dented inward. To cope with the above matter, various reinforcing structures are employed for the wrapper plate and the end plate of the low pressure casing. One example of such reinforcing structure is disclosed in Japanese Patent Application Laid-Open Publication No. 2002-235505.
In a conventional turbine casing structure including the reinforcing structure disclosed in the above Publication, as shown in FIGS. 8 and 9, for example, a low pressure casing 1 is divided into an upper-half portion 3 and a lower-half portion 4 by a horizontal surface through which a rotor axis 2 passes, and an upper-half flange 5 and a lower-half flange 6 are coupled with each other by means of bolts.
The upper-half portion 3 of the low pressure casing 1 is formed in an approximately semi-columnar shape by a semi-cylindrical wrapper plate 8 extending in an axial direction of the rotor axis 2 and semi-circular end plates 7 disposed on both the ends of the wrapper plate 8, and the lower-half portion 4 is formed in an approximately rectangular prism shape by end plates 9 and side plates 10.
The wrapper plate 8 of the upper-half portion 3 of the low pressure casing 1 is provided with a wrapper plate reinforcing ribs 11 disposed on the outside surface thereof in a peripheral direction with respect to the rotor axis 2 so as to apply rigidity to the wrapper plate 8. Further, as shown in FIG. 8, a plurality of end plate reinforcing ribs 12, which extend in a radial direction (radially) about the rotor axis 2, are joined to the outside surface of the end plates 7 of the upper-half portion 3 by means of welding, for example, so as to also apply rigidity to the end plates 7. In addition to the above arrangement, the rigidity is also applied to the end plates 7 by joining a plurality of lattice-shaped end plate reinforcing ribs 13 to the outside surface of the end plates 7 by means of welding as shown in, for example, FIG. 9.
The low pressure casing 1 is supported to a base 15 by leg plates 14, and the lower-half flange 6 of the lower-half portion 4 of the low pressure casing 1 is coupled with the leg plates 14 through leg plate reinforcing ribs 16.
Further, pipe stays 17, 18 are disposed to the lower-half portion 4 of the low pressure casing 1 to prevent deformation of the end plates 9 and the side plates 10 as shown in FIGS. 8, 9, and 10. The pipe stays 17 are interposed between the end plates 9 and the side plates 10. The pipe stays 18 are interposed between both the ends plates 9 in confrontation with each other in an axial direction of the low pressure casing 1.
Although the portion under the upper-half flange 5 and the lower-half flange 6 is also deformed so as to be dented in the axial direction by an external pressure received by the end plates 7 of the upper-half portion 3, the pipe stays 18 in particular have an effect of preventing such deformation.
In contrast, as shown in FIG. 10, an upper-half center rib 20, which couples the inner surface of the wrapper plate 8, the inner surfaces of the end plates 7, and the upper portion of a cone 19 with each other, is disposed in the upper-half portion 3 of the low pressure casing 1 to thereby secure the rigidity of the wrapper plate 8, the end plates 7 and the cone 19. On the other hand, a lower-half center rib 21, which couples the inner surface of the end plates 9 with the lower portion of the cone 19, is disposed in the lower-half portion 4 so as to secure the rigidity of the end plates 9 and the cone 19.
In a recent technology, since the size of the low pressure casing 1 is increased in accordance with elongation of the length of a blade in the final stage of a steam turbine, an area of the low pressure casing 1 on which external pressure is received is increased. Accordingly, in a conventional structure in which reinforcing ribs are disposed, the end plates 7 of the upper-half portion 3 are bent and deformed in a direction shown by two-dot-and-dash-lines A of FIG. 10 about a portion at which the end plates 7 are jointed to the wrapper plate 8.
In addition, in a case where the wrapper plate 8 is dented and deformed in its entirety by atmospheric pressure in an inside direction shown by two-dot-and-dash-lines B of FIGS. 8 and 10 or bent and deformed in a direction shown by two-dot-and-dash-lines C of FIGS. 8 and 9 about a portion where the upper-half flange 5 of the upper-half portion 3 is fastened to the lower-half flange 6 of the lower-half portion 4, deformation of the low pressure casing 1 cannot be suppressed.
Furthermore, since an internal constructions such as the pipe stays 17, 18 disposed in the upper-half portion 3 and the lower-half portion 4 of the low pressure casing 1, the upper-half center rib 20 and the like are exposed to the gas flow exhausted from the blade in the final stage of the steam turbine, although they achieve a function for securing the rigidity of the low pressure casing 1, there is a possibility that the resistance of a discharge gas flow path is increased and the gas exhaust performance of the low pressure casing 1 is deteriorated.